The purpose of this project is to develop methods for gene mapping in eukaryotic cells and for cloning small fractions of a mammalian genome. Gene transfer is detected by isolation of colonies of biologically transformed cells in selective medium after incubation of isolated metaphase chromosomes with mutant recipient cells deficient in HPRT or TK. The gene product is further characterized as chromosomal donor species. The method currently is most useful for determining close gene linkages. Nucleic acid hybridization is employed to determine the size of the transferred chromosome fragment (transgenome). The transgenome can be stabilized by integration into the chromosomal DNA of the recipient cell, and multiple integration sites exist.